This invention relates to a process for treating a cyanic liquid containing copper(I) cyanide complex ion.
When industrial effluents containing copper(I) cyanide complex ion are discharged, CN groups of copper(I) cyanide complex ion should be thoroughly decomposed prior to the discharge for prevention of environmental pollution. Hitherto, the CN group of copper(I) cyanide complex ion has been generally decomposed by a so-called alkali-chlorine process. This conventional process comprises adding sodium hypochlorite in an amount corresponding to the concentration of the copper(I) cyanide complex ion to the liquid to be treated while maintaining the pH of the liquid at a certain range during the treatment and decomposing the CN groups into carbon dioxide and nitrogen. This process is usually adopted in the treatment of a cyanic liquid containing a CN concentration of 2,000 ppm or less, especially under 1,000 ppm. When the cyanic liquid has a high CN concentration such as 10,000 ppm or higher, the treatment time is much longer and the amount of sodium hypochlorite is enormous so that it is practically impossible to thoroughly decompose the CN groups of copper(I) cyanide complex in a single batch. For treatment of the cyanic liquid having a high CN concentration such as 10,000 ppm or higher by the above conventional process attaining a decomposition rate of 90% or more requires diluting the cyanic liquid to 2,000 ppm or less and then subjecting the diluted liquid to treatment in several batches. These operations naturally make the working less efficient.
In order to overcome the difficulties of the conventional process, there has been proposed an improved process which comprises adding an aqueous solution of at least one ferrous salt selected from the group consisting of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous acetate, ammonium ferrous sulfate and ferrous iodide to a cyanic liquid containing a copper(I) cyanide complex ion and heating the liquid to 130.degree. C. or higher (cf. Japanese Patent Publication No. 20036/1982). By the improved process, the cyanic liquid having a CN concentration of from several ppm to 100,000 ppm can be effectively treated in a single batch without dilution of the liquid. Since, however, the above improved process requires a ferrous salt for decomposition of the CN groups of copper(I) cyanide complex ion, a sludge containing copper and iron is formed in the treated liquid, and the increased amount of the sludge tends to accumulate in a discharge pipe and makes the separation of the sludge from the liquid troublesome. It is particularly disadvantageous that the recovery of copper becomes difficult, because the sludge contains iron in addition to copper.